This disclosure relates to an acoustic device having an electro-acoustic transducer mounted to a passive radiator diaphragm.
A major problem in making a loudspeaker system for low frequency reproduction is to obtain a high output at the low frequencies while limiting loudspeaker cone excursion to reasonable limits within a displacement region relatively free from audible distortion sufficiently limited so that the cost of making this region is not excessive.
Many prior art low frequency speaker systems comprise a simple woofer with no enclosure, as in television and radio sets and some public address systems. A difficulty with these systems is that there is no means for preventing the radiation from the back of the speaker from canceling the radiation from the front. Such a system has very large cone excursions at low frequencies if they attempt of produce low bass.
One prior art approach for reducing back radiation is to place the loudspeaker driver in a closed box to form what is often called an acoustic suspension system. An acoustic suspension system provides a reactance against which the loudspeaker driver works, limiting the excursion and also preventing the radiation from the back of the loudspeaker from canceling that from the front.
A ported system is one prior art approach to improving upon the acoustic suspension system. A ported system typically includes a woofer in the enclosure and a port tube serving as a passive radiating means. The air in the port tube provides an acoustic mass that allows system design with an extra reactance which can be used to tailor the frequency response at the low end. A ported system is characterized by a resonance (port resonance) at which the mass of air in the port reacts with the volume of air in the cabinet to create a resonance at which the cone excursion of the loudspeaker is minimized. A ported system exhibits improved sensitivity at port resonance and decreased cone excursion, thereby minimizing distortion. The result of the improved sensitivity at port resonance is frequently an extension of the lower cutoff frequency of the loudspeaker to a lower value.
U.S. Pat. No. 4,549,631 describes a ported loudspeaker system which has an enclosure with a baffle that divides the interior into first and second subchambers. Each subchamber has a port tube that couples the subchamber to the region outside of the enclosure. The dividing baffle carries a woofer. The result of this arrangement having two subchambers and two port tubes is to lower the cone excursion in the low frequency region from that which could be obtained with a standard ported system and also to provide an additional parameter value that may be adjusted for maximizing response in the low frequency region.
While ported enclosures may be suitable for larger systems, they may not be as practical for smaller, portable systems. In that regard, another acoustic element for extending low frequency cutoff of a speaker system is a passive radiator. Passive radiators are typically employed where extending low frequency range is desired in smaller, e.g., portable, speaker systems. However, merely replacing the ports of the dual chamber design of the '631 patent with passive radiators could have undesirable consequences, e.g., unbalanced forces on the enclosure. The result could be undesirable movement or vibration of the enclosure. This is not an issue with the ports because there the moving masses, which are just plugs of air trapped in the ports, are small.